FIG. 9 illustrates a conventional servo accelerometer including a pendulum part made of quartz glass, which is a low thermal expansion material. In the conventional servo accelerometer, magnetic yokes 91, 92 made of invar, which is also a low thermal expansion material, support the pendulum part 93 to prevent thermal expansion strains. However, since invar has a low Curie temperature, magnetic saturation occurs in a high-temperature range, reducing the saturation flux density applied to a torquer coil 94. As a result, a temperature characteristic of the scale factor of the servo accelerometer becomes nonlinear and accordingly the accuracy of acceleration measurement can disadvantageously decrease.
In a servo accelerometer described in Japanese Patent Application Laid-Open No. 8-292208 (hereinafter referred to as Patent literature 1), magnetic reinforcing plates 97, 98 having a higher saturation flux density than cup-shaped magnetic yokes 91, 92 are provided on the outer bottoms of the magnetic yokes 91, 92 in order to reinforce magnetic circuits formed by magnets 95, 96, respectively.